Transmitters in many radio-based telecommunications and broadcast systems must transmit wide bandwidth modulated signals without adding too much distortion or noise. Generally, components in the entire signal processing chain, from the digital forming of baseband signals through analogue mixers and amplifiers to the final power amplification stage, add noise and distortion to the signal.
Noise that is inside the bandwidth of interest, i.e. the carrier bandwidth or potential carrier bandwidth, is controlled by having sufficient numbers of bits in the digital signals, good precision in the digital to analogue converters, and low noise components in the analogue chain. Noise that is outside the bandwidth of the carrier is generally reduced by filtering. Filtering is generally less costly and has less impact on efficiency if it is performed early in the processing chain, while filtering after final power amplification is especially costly, since at this point any insertion loss of a filter will have maximum effect on the efficiency of the transmitter.
Non-linear distortion or intermodulation is relatively large where the signal is large and when high efficiency is desired. This usually means that the final amplification stages, and especially the power amplifier, will distort the signal the most. A general band-limited carrier signal, i.e. multicarrier, multiuser, or, in general, an amplitude modulated signal, that is non-linearly distorted has a wider bandwidth than the original signal due to distortion sidebands. These distortion sidebands reduce the performance of the system, and are thus limited by standards and regulations to be below some specified level.
In order to counteract distortion in the final amplification stage, the input signal to the power amplifier is often predistorted. The predistortion can take place at digital or analogue baseband, at an intermediate frequency, or at the final radio frequency. Predistortion generally increases the bandwidth of the signal, similarly to actual distortion. Due to this bandwidth increase, noise close to the signal that occurs after the predistortion stage can no longer be filtered out in the early stages of the processing chain, since this would mean that important components of the predistorted signal would also be filtered out and not reach the power amplifier.
Predistortion can effectively reduce the distortion inside and close to the carriers. To have a large distortion-free bandwidth, the bandwidth of the predistortion signal must generally also be large. This can be costly, so instead a filter is usually applied after the power amplifier, to reduce noise and distortion further away from the carriers. Since such a filter is harder to build and/or has larger loss if a narrower transition from pass- to stopband is required, a trade-off exists between the bandwidth of the predistortion circuitry and subsequent circuits, and the transition width of the filter.
In addition to the need for filtering to achieve a pure transmitted signal, many systems that utilize frequency division duplex, i.e. different frequencies for transmit and receive signals, have a limit on the amount of noise and distortion that may leak from the transmitter to the receiver of the same equipment.
This leakage may be high if the antennas for transmitter and receiver are close to each other, or if one and the same antenna is used for both transmit and receive, which is otherwise economical. The leakage can be counteracted either by applying more filtering in the transmit branch than would otherwise be required by the standard or regulations, or by additional filtering in the receive branch.
US patent application 2005/0200422 describes a traditional feedforward distortion cancelling system in which an additional loop, dedicated to receiver noise reduction, includes a tuneable delay line and possibly a filter.
U.S. Pat. No. 7,058,368 describes a traditional feedforward system for receiver (or general) noise cancellation, with a cancellation loop which includes a filter.
These prior art solutions may improve upon a solution that uses only filters, since filters that achieve high stopband rejection also have high insertion loss in the passband. A combination of filtering with one of the prior art feedforward solutions can thus have lower insertion loss than a standalone filter.
The solutions proposed by US patent application 2005/0200422 and U.S. Pat. No. 7,058,368 have established that feedforward noise reduction can be beneficial both for reducing noise leakage to the receive band when co-locating transmitters and receivers and for reducing the transmission of noise outside the carrier band.
However, the insertion loss and/or the power consumption will still be high with these systems, so that the total system efficiency will be low. In addition, the solutions proposed by both of these documents result in a circuit complexity which is also rather high.